Recently, an electric vehicle (i.e., EV) and a hybrid vehicle (i.e., HV) have a structure for generating a small noise. Thus, it is difficult for a pedestrian to find these vehicles approaching the pedestrian. In order to increase a recognition degree of the vehicle disposed near and around the pedestrian or the like, the vehicle approach alert device for generating artificial engine sound is mounted on these vehicles (see, for example, Patent Literature 1).
The vehicle approach alert device outputs an approach alert sound such as a pseudo travelling sound via a speaker. When a sound pressure of an output from the speaker is large, a noise becomes a problem although a warning efficiency increases. On the other hand, when the sound pressure of then output from the speaker is small, the warning efficiency decreases although the noise is reduced. Accordingly, the sound pressure of then output from the speaker is set with a certain level for satisfying both the noise reduction and the warning efficiency.
However, the vehicle approach alert device is mounted on the vehicle, so that the speaker is influenced by environmental temperature around the vehicle. Specifically, the speaker is arranged on an outside of a compartment such as in an engine room. The usage temperature range in the engine room is very wide (a width of the range is 150° C.) such as in a range between −40° C. and 110° C. Thus, the influence to the speaker becomes large. For example, material of a voice coil in the speaker is copper. A temperature coefficient of a resistivity of copper is about 4000 ppm/° C. Thus, the impedance of the voice coil is changed about 60% within the temperature range of 150° C. Thus, the sound pressure of the output from the speaker is shown by a following equation. Thus, the sound pressure is reduced when the temperature increases. In the temperature range of 150° C., the sound pressure is simply reduced by 4 dB according to only the temperature change.1/(SPEAKER IMPEDANCE)∝(SPEAKER CURRENT)∝(SOUND PRESSURE OF SPEAKER OUTPUT)  (Equation 1)
Specifically, when a mounting position of the speaker is in the engine room, the sound pressure of sound from the speaker is reduced since the temperature in the engine room largely increases after and before driving the vehicle. Thus, the warning efficiency is reduced.